1. Field of the Invention
The invention relates to incandescent lamps, and more particularly, to incandescent lamps having filament inserts inserted into the open ends of coiled filaments for mounting the filament in the lamp.
2. Description of the Prior Art
A majority of incandescent lamps have coiled tungsten filaments with integral legs extending from the opposite ends of the filament coil for mounting the filament in the lamp. The legs may be clamped to conductive lead-ins extending into the lamp envelope or may extend through a sealed wall of the envelope to the exterior.
According to one method of filament manufacture, a series of filaments are formed by continuously winding a plurality of filament sections of desired length and pitch onto a molybdenum mandrel, the filament legs being turns of much lower pitch formed on the mandrel between the adjacent coiled filament sections. After annealing of the filament sections on the mandrel, the mandrel is severed between the filament sections and the molybdenum mandrel is removed, for example, in an acid bath. This yields individual filament coils with legs at each end, which legs may be further bent and/or cut to length for mounting the filament in the lamp envelope.
In the manufacture of certain incandescent lamps, however, the filament legs have proven to be problematic. For example, in halogen lamps where lamp cleanliness is of great importance, it is desirable to further heat the filaments on tungsten mandrels in an inert atmosphere before assembly in the lamp envelope to drive out impurities and to crystallize the filament coil. It has been found that the filament legs often are not heated to the same extent as the filament turns, resulting in a brittle transition between the leg and the respective filament end turn. This has resulted in an undesirable number of filament failures caused by the legs breaking off during further filament processing and sealing of the filaments in the lamp envelope.
In tubular infrared or flood lamps having an elongate filament arranged in a tubular envelope, the filament is provided during lamp processing with spiral wire supports to center the filament with respect to the envelope. The presence of filament legs in these lamps is also disadvantageous because the legs take up extra space and reduce the batch processing speed for winding the spirals on the filaments.
Thus, in various lamp types, it has proven economically feasible to make filament coils without legs, or to severe the legs, and at a later processing stage to use filament inserts inserted into the open coiled filament ends for supporting and electrically connecting the filament in the lamp envelope. The known filament inserts are threaded into open coiled ends of the filament. One form of insert, shown in FIG. 2, consists of a length of wire 33 having a coiled end with turns 35 of the same pitch as the filament coil 8'. The coiled end of the insert has an outside diameter selected such that it threads into the coiled end of the filament. Another known insert, shown in FIG. 3, consists of a rod 33' having a separate coil winding 35' also sized to thread into the inside of the filament coil.
Both of these insert designs have the disadvantage that they are labor intensive. It is difficult to begin the threading of the insert into the filament coil and the inserts must also be rotated a significant number of times to completely thread the insert into the filament coil, which is time consuming. Additionally, these inserts do not provide a simple mechanism for determining the insertion depth of the insert. Improper insertion depth can adversely effect the positioning and/or tension of the mounted filament. The latter has the additional disadvantage that two pieces of wire are required to form the insert, and the coil must be secured to the rod.